The present invention relates generally to multi-terminal DC power systems and, more particularly, to control schemes for such systems.
DC transmission is an attractive alternative to conventional ac transmission for offshore wind farms, solar farms located in remote areas, bulk and/or long-distance power transmission, and other applications. The control methods of the present invention are especially well suited (but not limited to) such DC transmission applications.
FIG. 1 shows a multi-terminal DC power system where multiple sources and loads (denoted as , , . . . , ) are connected together by a network (denoted as ). Each source or load is referred to as a terminal in this description. A source terminal receives power from a source (not shown) such as a wind turbine, a wind farm, an AC power system, or another DC power system, and feeds the received power into the DC network. A load terminal receives power from the DC network and feeds it to loads (not shown) in either DC or AC form. A source terminal will also be referred to as a sending terminal because it sends power to the DC network, and the two terms will be used interchangeably in this description. Similarly, a load terminal will also be referred to as a receiving terminal because it receives power from the DC network, and the two terms will also be used interchangeably in this description. The connections among the different terminals within the network  can be series, parallel, or a mix of the two (meshed).
In one embodiment of a multi-terminal DC power system, each source (sending) terminal receives power from a wind turbine power generator in an offshore wind farm, and multiple source terminals are connected in series in the DC network . The series-connected source terminals can be connected in parallel with other series-connected source terminals. Alternatively, multiple source terminals cm be connected in parallel first, and then in series with other parallel-connected source terminals. The power collected from the source terminals is fed through DC cables into onshore load (receiving) terminal which connects to an onshore (AC or DC) power network. Such a multi-terminal DC power system configuration be applied to integrate offshore wind farms with an onshore power grid, and it has the benefit that a medium or high voltage DC link can be established with the onshore power grid through series connection of low-voltage sending terminals, such that no additional power converter and associated offshore platform are required to boost the voltage.
In another embodiment of a multi-terminal DC power system, each sending or receiving terminal connects the DC network  to an AC power grid. The AC power grids connected by different terminals can also be interconnected by an AC network. The direction of power flow through each terminal may also reverse, so that a terminal can work as a sending terminal at one time and a receiving terminal at another time. One application of such a multi-terminal DC power system is in power distribution for large metropolitan areas that receive power from multiple AC in-feeds.
At the interface to the DC network, each terminal may behave like and be represented by a current-source converter (CSC) or a voltage-source converter (VSC). A CSC has an inductor in series at the interface to the DC network and appears as a current source to the DC network, white a VSC has a capacitor in parallel at the interface to the DC network and appears as a voltage source to the DC network. Note that the term current-source or voltage-source converter refers to the characteristics of the converter and does not imply that the terminal formed by the converter works as a source (sending) terminal in the DC network. A CSC or VSC in this invention is assumed to be able to support bidirectional power flow and can be operated as either a sending (source) or receiving (load) terminal. Moreover, this clarification is independent of the form of power that is interfaced with the DC network by such a converter—it can be poly-phase AC, single-phase AC, variable-frequency AC, DC, hybrid AC-DC, or any other form. Examples of a CSC and a VSC for interfacing three-phase AC with the DC network  include a line-commutated converter (LCC), and a pulse-width modulated (PWM) voltage-source converter (VSC), respectively.
Line-commutated converter (LCC) is a mature technology and is a common type of CSC for high-voltage DC (HVDC) applications. FIG. 2a depicts a 12-pulse LCC acting as an interface between three-phase AC and DC. The series inductor (L) at the interface to the DC network makes the converter behave like a current source to the DC network. It is the nature of LCC that the direction of DC interface current (i0) cannot change, but the polarity of the DC interface voltage (v0) can be reversed by changing the firing angle to reverse the direction of power flow.
FIG. 2b depicts a two-level PWM VSC. The parallel capacitor (C) at the interface to the DC network makes the converter behave like a voltage source to the DC network. It is the nature of a PWM VSC that the polarity of the DC interface voltage (v0) cannot be reversed, but the direction of current (i0) flow can be changed to change the direction of power flow.
There are many variations to the two-level PWM VSC depicted to FIG. 2b, including multilevel converters and modular multilevel converters (MMC). These converters am similar to the two-level VSC as far as their voltage-source characteristics at the interface to the DC network are concerned. Compared to LCC, PWM control has faster responses and cans operate with a unity, leading or lagging power factor at the AC interface. PWM control cars also be used in CSC to achieve similar improvements over conventional LCC CSC. The present invention is directed to the control of CSC and VSC for use in multi-terminal DC power systems regardless of their specific circuit constructions or device-level control (PWM or line-commutated.) The reference to specific converter types (such as LCC) for particular applications (such as HVDC transmission) in the following description is for the purpose of illustration only and does not limit the present invention to those specific types of converters or applications.
In conventional LCC-based HVDC systems, the current i0 at the DC interface of each terminal is the primary subject of control. The voltage (v0) the DC interface may also be controlled indirectly by considering it to be approximately equal to the voltage behind the inductor, that is, by ignoring the voltage across the inductor.
In a point-to-point HVDC system connecting two AC networks, for example, a simple control scheme is for the sending-terminal converter (rectifier) to control its output current while the receiving-terminal converter (inverter) maintains a constant voltage at some point of the DC link. The reference voltage and current of each terminal may also be made dependent of each other in order to implement such terminal characteristics as constant power or voltage or current droops.
Similar methods have been applied to VSC-based HVDC systems. In most cases, a VSC is simply used as replacement for a LCC and is controlled to provide similar characteristics at the DC interface.
The voltage and current at the DC interface of each terminal as defined in FIG. 1 depend on the converter as much as on the rest of the system. For example, the output current (i0) of the LCC in FIG. 2a is driven by the difference between the thyristor bridge voltage and the network voltage (v0) across the DC connection to the network. Similarly, the capacitor of the VSC in FIG. 2b is charged by the current flowing out of the converter switching circuit minus the network current (i0).
Therefore, the voltage and current at each terminal is strongly coupled to other terminals, such that their control must account for the dynamics of the entire DC network, which requires either central control or close coordination among different terminal controls. Fast communication is required in either case and control complexity increases exponentially with the number of terminals.
The requirement for communication bandwidth and the sensitivity to communication delays or fault also increases with the speed of control at each terminal, such that it is more difficult to implement such control for PWM-based systems.